Differences between acrylic and oil paint
The vehicle and binder of oil paints is linseed oil or another drying oil, whereas water serves as the vehicle for an emulsion of acrylic polymer that is the binder in acrylic paint. Thus, oil paint is said to be "oil-based", whereas acrylic paint is "water-based".
The main practical difference between most acrylics and oil paints is the inherent drying time. Oils allow for more time to blend colors and apply even glazes over underpaintings. This slow drying aspect of oil can be seen as an advantage for certain techniques, but in other regards it impedes the artist trying to work quickly. The fast evaporation of water from regular acrylic paint films can be slowed with the use of acrylic retarders. Retarders are generally glycol or glycerin-based additives. The addition of a retarder slows the evaporation rate of the water.
Oil paints may require the use of solvents such as mineral spirits or turpentine to thin the paint and clean up; these generally have some level of toxicity and are often found objectionable (relatively recently, water-miscible oil paints have been developed for artists' use). Oil paint films can become increasingly yellow and brittle with time and lose much of their flexibility in a few decades. Additionally, the rules of "fat over lean" must be employed to ensure the paint films are durable.
Oil paint has a higher pigment load than acrylic paint. As linseed oil has a smaller molecule than acrylic, oil paint is able to absorb substantially more pigment. Oil provides a different (less clear) refractive index than acrylic dispersions, imparting a unique "look and feel" to the resultant paint film. Not all pigments in oil are available in acrylic. Prussian blue has been recently added to the acrylic colors. Acrylic paints, unlike oil, may also be fluorescent.
Due to acrylic's more flexible nature and more consistent drying time between colors, the painter does not have to follow the "fat over lean" rule of oil painting, where more medium must be applied to each layer to avoid cracking. It usually takes between fifteen to twenty minutes for one to two layer of acrylic paint to dry. The rapid drying of the paint tends to discourage the blending of color and use of wet-in-wet technique as in oil painting. Even though acrylic retarders can slow drying time to several hours, it remains a relatively fast-drying medium, and the addition of too much acrylic retarder can prevent the paint from ever drying properly.
Meanwhile, acrylic paint is very elastic, which prevents cracking from occurring. Acrylic paint's binder is acrylic polymer emulsion; as this binder dries the paint remains flexible.
